1. Field of the Invention
The present invention relates to structures of semiconductor electronic products, and more particularly to a compound semiconductor device such as a semiconductor laser, a photoreceptor, an opto-electronic integrated circuit device and a high frequency electronic device.
2. Description of the Related Art
Conventionally, a heterojunction type compound semiconductor device such as a photoreceptor, a semiconductor laser, an optical device such as an optical integrated device and a high frequency electronic device is configured using a heterostructure formed by an epitaxial crystal which is lattice-matched to the substrate crystal, or by a strained epitaxial crystal in which the lattice constant is slightly mismatched.
(1) For the above heterostructure, since the binary crystal such as GaAs or InP is used as a substrate crystal, the kinds of the compound semiconductor alloys which can be used are limited when the lattice matching conditions or, in the case of the strained crystal, the elasticity strain limits, are taken into consideration. Thus, there are cases where it is difficult or impossible for the conditions required for the oscillating and light receiving wavelengths to be satisfied during the designing of a device such as a semiconductor laser or a light receiving element. If these conditions or limitations during the designing stage can be relaxed, there will be a great advantage in increasing a degree of freedom in the device designing.
(2) In fabricating an optically integrated device, it is desirable that the bandgap be made variable in the longitudinal direction of a waveguide and, in fabricating the waveguide, it is desirable that the bandgap and the refractive index be made variable in the direction orthogonal to the longitudinal direction of the waveguide within a surface of the crystal, that is, in the lateral direction of the waveguide. To this end, conventionally, there have been employed processes in which a part of the epitaxial layer grown is removed by etching, and a layer that has a different bandgap is newly and selectively grown. However, such processes are time consuming, and are often accompanied by problems associated with lattice defects in the crystal at re-grown interfaces. Thus, it would be advantageous if, by avoiding such processes, the bandgap variation and the refractive index variation without crystal defects can be achieved.